Platinum-catalyzed asymmetric hydrophosphination of activated olefins using the catalyst precursor Pt(R,R-Me-Duphos)(trans-stilbene) (1) gives chiral phosphines with control of stereochemistry at phosphorus or carbon centers. Stoichiometric reactions of 1 allow observation of P-H oxidative addition, diastereoselective olefin insertion, and reductive elimination steps, which make up the proposed catalytic cycle.Chiral phosphines, valuable ligands for metal-catalyzed asymmetric reactions, 1 are usually prepared either by resolution or by using a stoichiometric amount of a chiral auxiliary. 2 Surprisingly little work has been reported on metal-catalyzed asymmetric syntheses. 3 We report here that Pt-catalyzed asymmetric hydrophosphination of activated olefins 4 can be used to prepare chiral phosphines with control of stereochemistry at phosphorus or carbon. Although the enantiomeric excesses (ee's) available thus far are low, mechanistic understanding may allow further development of these new reactions.Scheme 1 shows a mechanism for Pt-catalyzed hydrophosphination, proposed on the basis of our previous studies. 5 After P-H oxidative addition, P-C bond formation occurs by selective insertion of the olefin into the Pt-P bond. Reductive elimination forms the product and regenerates Pt(0). Since the insertion step can be diastereoselective, 5,6 use of a chiral Pt catalyst could lead to enantio-enriched product. For example, a disubstituted olefin could give a phosphine (Scheme 2) with controlled stereochemistry at either of the alkene carAn important recent exception is Burk's synthesis of Duphos ligands, which relies on Ru-Binap-catalyzed asymmetric hydrogenation of -keto esters (Burk, M. J.; Feaster, J. E.; Nugent, W. A.; Harlow, R. L. J. Am. Chem. Soc. 1993, 115, 10125-10138).(4) Pringle and co-workers have shown that Pt-catalyzed hydrophosphination can be used to prepare functionalized phosphines. (a) Costa, E.; Pringle, P. G.; Worboys, K. Chem. Commun. 1998, 49-50. (b) Costa, E.; Pringle, P. G.; Smith, M. B.; Worboys, K. Nolan, S. P.; Porchia, M.; Sishta, C.; Marks, T. J. In Energetics of Organometallic Species; Martinho Simoes, J. A., Ed.; Kluwer: Dordrecht, 1992; pp 35-51. (5) (a) Wicht, D. K.; Kourkine, I. V.; Lew, B. M.; Nthenge, J. M.; Glueck, D. S. J. Am. Chem. Soc. 1997, 119, 5039-5040. (b) Wicht, D. K.; Kourkine, I. V.; Kovacik, I.; Glueck, D. S.; Concolino, T. E.; Yap, G. P. A.; Incarvito, C. D.; Rheingold, A. L. Organometallics 1999, 18, 5381-5394.Scheme 1. Proposed Mechanism for Pt-Catalyzed Hydrophosphination a a [Pt] ) Pt(diphosphine), X ) CN, CO2R, or other electronwithdrawing group. Scheme 2. Proposed Mechanism for Pt-Catalyzed Asymmetric Hydrophosphination of Disubstituted Alkenes a a [Pt] ) Pt(chiral diphosphine), X ) CN, CO2R, or other electron-withdrawing group. 950Organometallics 2000, 19,[950][951][952][953] 10.

The zerovalent complexes Pt(diphos)(trans-stilbene) (diphos = dppe, dppf, (S,S)-Chiraphos, R-Tol-Binap, (R,R)-Me-Duphos, (S,S)-Diop) were prepared by reduction of the corresponding Pt(diphos)Cl2 compounds. NMR and crystal structure data show that the trans-stilbene complexes with chiral diphosphines exist as a mixture of diastereomers differing in their binding of the enantiofaces of the prochiral olefin; for R-Tol-Binap, they can be separated by recrystallization. The complexes Pt((R,R)-Me-Duphos)Cl2 and Pt(diphos)(trans-stilbene) (diphos = dppf, (S,S)-Chiraphos, R-Tol-Binap, (S,S)-Diop) were structurally characterized by X-ray crystallography.

No abstract

A series of terminal Pt(II) phosphido complexes Pt(dppe)(Me)(PRR‘) (R = H; R‘ = Mes* (1), R‘ = Mes (2), R‘ = Ph (3), R‘ = Cy (4); R = R‘ = Mes (5); R = R‘ = Ph (6); R = R‘ = Cy (7); R = R‘ = Et (8); R = Ph, R‘ = i-Bu (9)) has been prepared by proton transfer from the appropriate phosphine to the methoxide ligand of Pt(dppe)(Me)(OMe) (10) (dppe = Ph2PCH2CH2PPh2; Mes* = 2,4,6-(t-Bu)3C6H2; Mes = 2,4,6-Me3C6H2; Cy = cyclo-C6H11). Complexes 1 and 2 were also made by deprotonation of the cations [Pt(dppe)(Me)(PH2Ar)][BF4] (Ar = Mes* (13); Ar = Mes (14)). For comparison to 1, the arylthiolate and aryloxide complexes Pt(dppe)(Me)(EMes*) (E = S (11); E = O (12)) were also prepared from 10. NMR studies of the proton-transfer equilibria between Pt(dppe)(Me)(X), Pt(dppe)(Me)(Y), and the acids HY and HX (see Bryndza, H. E.; Fong, L. K.; Paciello, R. A.; Tam, W.; Bercaw, J. E. J. Am. Chem. Soc. 1987, 109, 1444−1456 and Bryndza, H. E.; Domaille, P. J.; Tam, W.; Fong, L. K.; Paciello, R. A.; Bercaw, J. E. Polyhedron 1988, 7, 1441−1452) provide an approximate partial ranking of Pt−P bond strengths in this series:  Pt−PHPh > Pt−PHMes > Pt−PHMes*; Pt−PPh2 > Pt−PMes2. Complementary solution calorimetry investigations probe the role of entropic effects on the equilibria. Both steric and electronic factors appear to be important in controlling relative Pt−P bond strengths. The Pt−S bonds in 11 and Pt(dppe)(Me)(SPh) are stronger than the analogous Pt−P bonds in 1 and 3. Complexes 1 and 5·THF were structurally characterized by X-ray crystallography.

The acrylonitrile complexes Pt(diphos)(CH 2 CHCN) (diphos ) dppe (1), dcpe (2); dppe ) Ph 2 PCH 2 CH 2 PPh 2 , dcpe ) Cy 2 PCH 2 CH 2 PCy 2 , Cy ) cyclo-C 6 H 11 ) are catalyst precursors and, for some substrates, resting states, during addition of P-H bonds in primary and secondary phosphines across the CdC double bond of acrylonitrile (hydrophosphination). Oxidative addition of P-H bonds to related catalyst precursors gives the phosphido hydride complexes Pt(diphosAcrylonitrile does not insert into the Pt-H bond of these hydrides to give cyanoethyl ligands; the putative products, the phosphido complexes Pt(diphos 11)) were prepared independently and found to be stable to P-C reductive elimination. Instead, catalysis appears to occur by selective insertion of acrylonitrile into the Pt-P bond to yield the alkyl hydrides Pt(diphos)[CH(CN)CH 2 PRR′](H), followed by C-H reductive elimination and regeneration of 1 or 2. This insertion was observed directly in model methyl phosphido complexes M(dppe)-(Me)(PRR′) (M ) Pt, R ) H, R′ ) Mes* (12), R ) R′ ) Mes (13); M ) Pd, R ) H, R′ ) Mes* (17)), yielding M(dppe)[CH(CN)CH 2 PRR′](Me), (14, 15, 18). Similarly, treatment of Pt(dcpe)-(PHMes*)(H) (22) with acrylonitrile gives Pt(dcpe)[CH(CN)CH 2 PHMes*](H) (24) as a mixture of diastereomers; the isomeric Pt(dcpe)[PMes*(CH 2 CH 2 CN)](H) (25), which was prepared independently, was also observed during this reaction. Both 24 and 25 decompose in the presence of acrylonitrile to form Pt(dcpe)(CH 2 CHCN) (2) and PHMes*(CH 2 CH 2 CN) (3a). The C-H reductive elimination step was modeled by studies of Pt(dcpe)[CH(Me)(CN)](H) (26). Another isomer, Pt(dcpe)[CH(Me)(CN)](PHMes*) (29), which formally results from insertion of acrylonitrile into the Pt-H bond of 22, was formed by decomposition of complex 2 during catalysis. Complex 29 is inactive in catalysis but decomposes to partially regenerate the active catalyst 2. The cyanoethyl compounds Pt(dcpe)(CH 2 CH 2 CN)(PHMes*) (11), trans-Pt-(PPh 3 ) 2 (CH 2 CH 2 CN)(Br), and PMes 2 (CH 2 CH 2 CN) (23) were structurally characterized by X-ray crystallography.